The furnace/AC system at our house in Memphis is going flaky . It has
burned out the primary winding on 3 24V transformers ... The control
board has been replaced , it doesn't blow the transformer when calling
for heat , just AC .
Our son that lives there has left the cover over the condenser
relay/capacitor off after a start cap replacement , I'm wondering if
rust in the relay might be the cause of the problem . What really
puzzles me is why is it burning out the primary winding ? The fuse in
the board is intact , the last transformer lasted about 20-30 seconds
when we turned the AC on . Having already spent over a hundred bucks on
parts already , I'm not inclined to spend more until I can figure this
out .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell

On Fri, 12 Aug 2022 12:05:59 -0500, Snag <snag_one@msn.com> wrote:

> The furnace/AC system at our house in Memphis is going flaky . It has
>burned out the primary winding on 3 24V transformers ... The control
>board has been replaced , it doesn't blow the transformer when calling
>for heat , just AC .
> Our son that lives there has left the cover over the condenser
>relay/capacitor off after a start cap replacement , I'm wondering if
>rust in the relay might be the cause of the problem . What really
>puzzles me is why is it burning out the primary winding ? The fuse in
>the board is intact , the last transformer lasted about 20-30 seconds
>when we turned the AC on . Having already spent over a hundred bucks on
>parts already , I'm not inclined to spend more until I can figure this
>out .
>

Just a quick google search - links below seem to suggest
shorted wires somewhere < ? >

https://hvac-talk.com/vbb/threads/73109-why-on-the-primary-side

https://www.fixya.com/support/t5180226-ve_got_24_volt_transformer_amp_fuse

https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/

John T.

On 8/12/2022 12:24 PM, hubops@ccanoemail.com wrote:
> On Fri, 12 Aug 2022 12:05:59 -0500, Snag <snag_one@msn.com> wrote:
>
>> The furnace/AC system at our house in Memphis is going flaky . It has
>> burned out the primary winding on 3 24V transformers ... The control
>> board has been replaced , it doesn't blow the transformer when calling
>> for heat , just AC .
>> Our son that lives there has left the cover over the condenser
>> relay/capacitor off after a start cap replacement , I'm wondering if
>> rust in the relay might be the cause of the problem . What really
>> puzzles me is why is it burning out the primary winding ? The fuse in
>> the board is intact , the last transformer lasted about 20-30 seconds
>> when we turned the AC on . Having already spent over a hundred bucks on
>> parts already , I'm not inclined to spend more until I can figure this
>> out .
>>
>
>
> Just a quick google search - links below seem to suggest
> shorted wires somewhere < ? >
>
> https://hvac-talk.com/vbb/threads/73109-why-on-the-primary-side
>
> https://www.fixya.com/support/t5180226-ve_got_24_volt_transformer_amp_fuse
>
> https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/
>
> John T.
>

I should have mentioned that we did find some bare/shorted wires and
repaired them . The son is/was supposed to do a check to be sure we
corrected the short , but I doubt he actually did what I asked . This is
the same son who burned up the trans in my truck because it was too much
trouble to monitor the oil level - after he was told that the rear seal
was leaking a little and to watch it . That cost me a motor too , caused
by a failed oil pressure sensor that was damaged during the trans swap .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell

On Friday, August 12, 2022 at 1:41:26 PM UTC-4, Snag wrote:
> On 8/12/2022 12:24 PM, hub...@ccanoemail.com wrote:
> > On Fri, 12 Aug 2022 12:05:59 -0500, Snag <snag...@msn.com> wrote:
> >
> >> The furnace/AC system at our house in Memphis is going flaky . It has
> >> burned out the primary winding on 3 24V transformers ... The control
> >> board has been replaced , it doesn't blow the transformer when calling
> >> for heat , just AC .
> >> Our son that lives there has left the cover over the condenser
> >> relay/capacitor off after a start cap replacement , I'm wondering if
> >> rust in the relay might be the cause of the problem . What really
> >> puzzles me is why is it burning out the primary winding ? The fuse in
> >> the board is intact , the last transformer lasted about 20-30 seconds
> >> when we turned the AC on . Having already spent over a hundred bucks on
> >> parts already , I'm not inclined to spend more until I can figure this
> >> out .
> >>
> >
> >
> > Just a quick google search - links below seem to suggest
> > shorted wires somewhere < ? >
> >
> > https://hvac-talk.com/vbb/threads/73109-why-on-the-primary-side
> >
> > https://www.fixya.com/support/t5180226-ve_got_24_volt_transformer_amp_fuse
> >
> > https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/
> >
> > John T.
> >
> I should have mentioned that we did find some bare/shorted wires and
> repaired them . The son is/was supposed to do a check to be sure we
> corrected the short , but I doubt he actually did what I asked . This is
> the same son who burned up the trans in my truck because it was too much
> trouble to monitor the oil level - after he was told that the rear seal
> was leaking a little and to watch it . That cost me a motor too , caused
> by a failed oil pressure sensor that was damaged during the trans swap .
> --

Well, it seems to me that you have found the root cause of the problem.

Maybe you should throw some dollars towards fixing your son. ;-)

In alt.home.repair, on Fri, 12 Aug 2022 12:05:59 -0500, Snag
<snag_one@msn.com> wrote:

> The furnace/AC system at our house in Memphis is going flaky . It has
>burned out the primary winding on 3 24V transformers ... The control
>board has been replaced , it doesn't blow the transformer when calling
>for heat , just AC .
> Our son that lives there has left the cover over the condenser

And I gather you don't live there. Problem solved! You probably have
AC where you live, so no AC for your son until your son figures out
what's wrong. He can start by doing the checks for shorts you told him
to do.

I do appreciate your desire to help him. My brother is in the same
situation. I guess you could send him the links that hubops found.

>relay/capacitor off after a start cap replacement , I'm wondering if
>rust in the relay might be the cause of the problem . What really
>puzzles me is why is it burning out the primary winding ? The fuse in
>the board is intact , the last transformer lasted about 20-30 seconds
>when we turned the AC on . Having already spent over a hundred bucks on
>parts already , I'm not inclined to spend more until I can figure this
>out .

On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
> The furnace/AC system at our house in Memphis is going flaky . It has
> burned out the primary winding on 3 24V transformers ... The control
> board has been replaced , it doesn't blow the transformer when calling
> for heat , just AC .
> Our son that lives there has left the cover over the condenser
> relay/capacitor off after a start cap replacement , I'm wondering if
> rust in the relay might be the cause of the problem . What really
> puzzles me is why is it burning out the primary winding ? The fuse in
> the board is intact , the last transformer lasted about 20-30 seconds
> when we turned the AC on . Having already spent over a hundred bucks on
> parts already , I'm not inclined to spend more until I can figure this
> out .
> --
> Snag
> “Free speech is my right to say what you don’t
> want to hear.” -George Orwell

Maybe you should unhook the wires right at the relay coil. See what happens when you turn the
AC on. Can you hook the relay to 24 volts from another source? Fuse it first.

On 8/12/2022 1:51 PM, Dean Hoffman wrote:
> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>> The furnace/AC system at our house in Memphis is going flaky . It has
>> burned out the primary winding on 3 24V transformers ... The control
>> board has been replaced , it doesn't blow the transformer when calling
>> for heat , just AC .
>> Our son that lives there has left the cover over the condenser
>> relay/capacitor off after a start cap replacement , I'm wondering if
>> rust in the relay might be the cause of the problem . What really
>> puzzles me is why is it burning out the primary winding ? The fuse in
>> the board is intact , the last transformer lasted about 20-30 seconds
>> when we turned the AC on . Having already spent over a hundred bucks on
>> parts already , I'm not inclined to spend more until I can figure this
>> out .
>> --
>> Snag
>> “Free speech is my right to say what you don’t
>> want to hear.” -George Orwell
>
> Maybe you should unhook the wires right at the relay coil. See what happens when you turn the
> AC on. Can you hook the relay to 24 volts from another source? Fuse it first.
>

After repairing the bare wires and replacing the 2nd transformer I
checked voltage at the AC condenser unit before hooking the wires to the
relay . 24V on the nose . I don't really think I have another short in
the control wire , I suspect excessive current draw on the relay because
there is rust in there preventing it from completing the magnetic
circuit . But I've still never seen the primary winding blow like these
did . I'm riding down tomorrow (on my Harley) to help install a window
unit I bought and had delivered . Gotta make sure it's done right ...
he's too likely to take short cuts .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell

On Friday, August 12, 2022 at 4:59:01 PM UTC-5, Snag wrote:
> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
> > On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
> >> The furnace/AC system at our house in Memphis is going flaky . It has
> >> burned out the primary winding on 3 24V transformers ... The control
> >> board has been replaced , it doesn't blow the transformer when calling
> >> for heat , just AC .
> >> Our son that lives there has left the cover over the condenser
> >> relay/capacitor off after a start cap replacement , I'm wondering if
> >> rust in the relay might be the cause of the problem . What really
> >> puzzles me is why is it burning out the primary winding ? The fuse in
> >> the board is intact , the last transformer lasted about 20-30 seconds
> >> when we turned the AC on . Having already spent over a hundred bucks on
> >> parts already , I'm not inclined to spend more until I can figure this
> >> out .
> >> --
> >> Snag
> >> “Free speech is my right to say what you don’t
> >> want to hear.” -George Orwell
> >
> > Maybe you should unhook the wires right at the relay coil. See what happens when you turn the
> > AC on. Can you hook the relay to 24 volts from another source? Fuse it first.
> >
> After repairing the bare wires and replacing the 2nd transformer I
> checked voltage at the AC condenser unit before hooking the wires to the
> relay . 24V on the nose . I don't really think I have another short in
> the control wire , I suspect excessive current draw on the relay because
> there is rust in there preventing it from completing the magnetic
> circuit . But I've still never seen the primary winding blow like these
> did . I'm riding down tomorrow (on my Harley) to help install a window
> unit I bought and had delivered . Gotta make sure it's done right ...
> he's too likely to take short cuts .
> --
> Snag
> “Free speech is my right to say what you don’t
> want to hear.” -George Orwell

There are some specs here of a 24 VAC relay for what good it does . 8.7 ohms coil resistance. it's supposed to draw 375mA. There are some inline automotive type fuses that might help protect things upstream.
<https://www.mouser.com/ProductDetail/TE-Connectivity-PB/S89R11AAC1-24?qs=ngFyHZor3%252BcQ7N5VzGuRlw%3D%3D>

In alt.home.repair, on Fri, 12 Aug 2022 13:24:22 -0400,
hubops@ccanoemail.com wrote:

>On Fri, 12 Aug 2022 12:05:59 -0500, Snag <snag_one@msn.com> wrote:
>
>> The furnace/AC system at our house in Memphis is going flaky . It has
>>burned out the primary winding on 3 24V transformers ... The control
>>board has been replaced , it doesn't blow the transformer when calling
>>for heat , just AC .
>> Our son that lives there has left the cover over the condenser
>>relay/capacitor off after a start cap replacement , I'm wondering if
>>rust in the relay might be the cause of the problem . What really
>>puzzles me is why is it burning out the primary winding ? The fuse in
>>the board is intact , the last transformer lasted about 20-30 seconds
>>when we turned the AC on . Having already spent over a hundred bucks on
>>parts already , I'm not inclined to spend more until I can figure this
>>out .
>>
>
>
>Just a quick google search - links below seem to suggest
> shorted wires somewhere < ? >
>
>https://hvac-talk.com/vbb/threads/73109-why-on-the-primary-side
>
>https://www.fixya.com/support/t5180226-ve_got_24_volt_transformer_amp_fuse
>
>https://arnoldservice.com/problem-my-24-volt-transformer-continues-to-burn-up-this-is-the-second-transformer-what-could-be-the-problem/
>
> John T.

After reading these, I remember that my furnace/ac transformer blew out
the first July 4 weekend I lived here. It only happened once, maybe
because I bought a bigger transformer. AFAicr I didn't check if it was
the primary or seconddary, only that the output was zero. I went to to
a heating supply store and tried to buy an exact replacement xformer.
He woudl have sold me a 250 dollar control board, but when I balked he
offered me a 10 dollar xformer that I had to moount elsewhee in the
cabinet. It worked for 30 years.

I don't see how shorted wires can be the problem, I think the xiformer
draws so much it overheats and burns out, like one of the posters in one
of the links said. Buy a bigger transformer. Everyone knows bigger is
better. Ask Tim Taylor.

On Friday, August 12, 2022 at 5:19:16 PM UTC-5, Dean Hoffman wrote:
> On Friday, August 12, 2022 at 4:59:01 PM UTC-5, Snag wrote:
> > On 8/12/2022 1:51 PM, Dean Hoffman wrote:
> > > On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
> > >> The furnace/AC system at our house in Memphis is going flaky . It has
> > >> burned out the primary winding on 3 24V transformers ... The control
> > >> board has been replaced , it doesn't blow the transformer when calling
> > >> for heat , just AC .
> > >> Our son that lives there has left the cover over the condenser
> > >> relay/capacitor off after a start cap replacement , I'm wondering if
> > >> rust in the relay might be the cause of the problem . What really
> > >> puzzles me is why is it burning out the primary winding ? The fuse in
> > >> the board is intact , the last transformer lasted about 20-30 seconds
> > >> when we turned the AC on . Having already spent over a hundred bucks on
> > >> parts already , I'm not inclined to spend more until I can figure this
> > >> out .
> > >> --
> > >> Snag
> > >> “Free speech is my right to say what you don’t
> > >> want to hear.” -George Orwell
> > >
> > > Maybe you should unhook the wires right at the relay coil. See what happens when you turn the
> > > AC on. Can you hook the relay to 24 volts from another source? Fuse it first.
> > >
> > After repairing the bare wires and replacing the 2nd transformer I
> > checked voltage at the AC condenser unit before hooking the wires to the
> > relay . 24V on the nose . I don't really think I have another short in
> > the control wire , I suspect excessive current draw on the relay because
> > there is rust in there preventing it from completing the magnetic
> > circuit . But I've still never seen the primary winding blow like these
> > did . I'm riding down tomorrow (on my Harley) to help install a window
> > unit I bought and had delivered . Gotta make sure it's done right ...
> > he's too likely to take short cuts .
> > --
> > Snag
> > “Free speech is my right to say what you don’t
> > want to hear.” -George Orwell
> There are some specs here of a 24 VAC relay for what good it does . 8.7 ohms coil resistance. it's supposed to draw 375mA. There are some inline automotive type fuses that might help protect things upstream.
> <https://www.mouser.com/ProductDetail/TE-Co
There's another one here that might be closer to what you'd want. A similar 120 volt version of this relay was used in pivot panels for awhile.
<https://www.zoro.com/dayton-open-power-relay-8-pin-24vac-dpdt-5x846/i/G2626872/#specifications>
Amazon has relays for sale. I didn't look at Ebay

On 8/12/2022 5:53 PM, Dean Hoffman wrote:
> On Friday, August 12, 2022 at 5:19:16 PM UTC-5, Dean Hoffman wrote:
>> On Friday, August 12, 2022 at 4:59:01 PM UTC-5, Snag wrote:
>>> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
>>>> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>>>>> The furnace/AC system at our house in Memphis is going flaky . It has
>>>>> burned out the primary winding on 3 24V transformers ... The control
>>>>> board has been replaced , it doesn't blow the transformer when calling
>>>>> for heat , just AC .
>>>>> Our son that lives there has left the cover over the condenser
>>>>> relay/capacitor off after a start cap replacement , I'm wondering if
>>>>> rust in the relay might be the cause of the problem . What really
>>>>> puzzles me is why is it burning out the primary winding ? The fuse in
>>>>> the board is intact , the last transformer lasted about 20-30 seconds
>>>>> when we turned the AC on . Having already spent over a hundred bucks on
>>>>> parts already , I'm not inclined to spend more until I can figure this
>>>>> out .
>>>>> --
>>>>> Snag
>>>>> “Free speech is my right to say what you don’t
>>>>> want to hear.” -George Orwell
>>>>
>>>> Maybe you should unhook the wires right at the relay coil. See what happens when you turn the
>>>> AC on. Can you hook the relay to 24 volts from another source? Fuse it first.
>>>>
>>> After repairing the bare wires and replacing the 2nd transformer I
>>> checked voltage at the AC condenser unit before hooking the wires to the
>>> relay . 24V on the nose . I don't really think I have another short in
>>> the control wire , I suspect excessive current draw on the relay because
>>> there is rust in there preventing it from completing the magnetic
>>> circuit . But I've still never seen the primary winding blow like these
>>> did . I'm riding down tomorrow (on my Harley) to help install a window
>>> unit I bought and had delivered . Gotta make sure it's done right ...
>>> he's too likely to take short cuts .
>>> --
>>> Snag
>>> “Free speech is my right to say what you don’t
>>> want to hear.” -George Orwell
>> There are some specs here of a 24 VAC relay for what good it does . 8.7 ohms coil resistance. it's supposed to draw 375mA. There are some inline automotive type fuses that might help protect things upstream.
>> <https://www.mouser.com/ProductDetail/TE-Co
> There's another one here that might be closer to what you'd want. A similar 120 volt version of this relay was used in pivot panels for awhile.
> <https://www.zoro.com/dayton-open-power-relay-8-pin-24vac-dpdt-5x846/i/G2626872/#specifications>
> Amazon has relays for sale. I didn't look at Ebay
>
>
>

I still have a cash account at a Memphis wholesale heating and air
supply house from my handyman days .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell

On 8/12/2022 3:58 PM, Snag wrote:
> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
>> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>>> The furnace/AC system at our house in Memphis is going flaky . It has
>>> burned out the primary winding on 3 24V transformers ... The control
>>> board has been replaced , it doesn't blow the transformer when calling
>>> for heat , just AC .
>>> Our son that lives there has left the cover over the condenser
>>> relay/capacitor off after a start cap replacement , I'm wondering if
>>> rust in the relay might be the cause of the problem . What really
>>> puzzles me is why is it burning out the primary winding ? The fuse in
>>> the board is intact , the last transformer lasted about 20-30 seconds
>>> when we turned the AC on . Having already spent over a hundred bucks on
>>> parts already , I'm not inclined to spend more until I can figure this
>>> out .

If the transformer connects to the thermostat it is "class 2", which is
limited power. I never tired shorting one, but I can't imagine only
lasting 30 seconds if shorted.

I haven't seen an HVAC transformer larger than 40 VA. At 40VA that is
1/3 A on the primary (plus something for inefficiency). Would be more
for a short, but it is limited power. Doesn't seem like much to burn out
the primary.

40VA - at 24V that would supply 1.7A to equipment (more if shorted).
Measure current from transformer, particularly when AC is started
(temporarily)? If you have a clamp-on ammeter, if you run the wire
around and through the window 10 times it will read 10 times the
current. You could get some similar ideas with low value fuses.

Measure the transformer voltage when you start the AC?

>>> --
>>> Snag
>>> “Free speech is my right to say what you don’t
>>> want to hear.” -George Orwell
>>
>>       Maybe you should unhook the wires right at the relay coil.   See
>> what happens when you turn the
>> AC on.  Can you hook the relay to 24 volts from another source?  Fuse
>> it first.
>>
>
>   After repairing the bare wires and replacing the 2nd transformer I
> checked voltage at the AC condenser unit before hooking the wires to the
> relay . 24V on the nose . I don't really think I have another short in
> the control wire , I suspect excessive current draw on the relay because
> there is rust in there preventing it from completing the magnetic
> circuit . But I've still never seen the primary winding blow like these
> did . I'm riding down tomorrow (on my Harley) to help install a window
> unit I bought and had delivered . Gotta make sure it's done right ...
> he's too likely to take short cuts .

If the compressor relay closes there is a continuous magnetic path and
the relay current is limited by the inductance. If the relay can't close
there is an air gap in the magnetic path and the inductance will be less
- higher coil current. (AC relays close faster because of high coil
current until closed.)

On 8/14/2022 1:18 PM, bud-- wrote:
> On 8/12/2022 3:58 PM, Snag wrote:
>> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
>>> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>>>> The furnace/AC system at our house in Memphis is going flaky . It has
>>>> burned out the primary winding on 3 24V transformers ... The control
>>>> board has been replaced , it doesn't blow the transformer when calling
>>>> for heat , just AC .
>>>> Our son that lives there has left the cover over the condenser
>>>> relay/capacitor off after a start cap replacement , I'm wondering if
>>>> rust in the relay might be the cause of the problem . What really
>>>> puzzles me is why is it burning out the primary winding ? The fuse in
>>>> the board is intact , the last transformer lasted about 20-30 seconds
>>>> when we turned the AC on . Having already spent over a hundred bucks on
>>>> parts already , I'm not inclined to spend more until I can figure this
>>>> out .
>
> If the transformer connects to the thermostat it is "class 2", which is
> limited power. I never tired shorting one, but I can't imagine only
> lasting  30 seconds if shorted.
>
> I haven't seen an HVAC transformer larger than 40 VA. At 40VA that is
> 1/3 A on the primary (plus something for inefficiency). Would be more
> for a short, but it is limited power. Doesn't seem like much to burn out
> the primary.
>
> 40VA  - at 24V that would supply 1.7A to equipment (more if shorted).
> Measure current from transformer, particularly when AC is started
> (temporarily)?  If you have a clamp-on ammeter, if you run the wire
> around and through the window 10 times it will read 10 times the
> current. You could get some similar ideas with low value fuses.
>
> Measure the transformer voltage when you start the AC?

WE installed a window unit yesterday ... I'm not planning on going
back up to that attic until the weather cools . And then probably only
to insure the furnace is going to work .
>
>>>> --
>>>> Snag
>>>> “Free speech is my right to say what you don’t
>>>> want to hear.” -George Orwell
>>>
>>>       Maybe you should unhook the wires right at the relay coil.
>>> See what happens when you turn the
>>> AC on.  Can you hook the relay to 24 volts from another source?  Fuse
>>> it first.
>>>
>>
>>    After repairing the bare wires and replacing the 2nd transformer I
>> checked voltage at the AC condenser unit before hooking the wires to
>> the relay . 24V on the nose . I don't really think I have another
>> short in the control wire , I suspect excessive current draw on the
>> relay because there is rust in there preventing it from completing the
>> magnetic circuit . But I've still never seen the primary winding blow
>> like these did . I'm riding down tomorrow (on my Harley) to help
>> install a window unit I bought and had delivered . Gotta make sure
>> it's done right ... he's too likely to take short cuts .
>
> If the compressor relay closes there is a continuous magnetic path and
> the relay current is limited by the inductance. If the relay can't close
> there is an air gap in the magnetic path and the inductance will be less
> - higher coil current.  (AC relays close faster because of high coil
> current until closed.)

This is exactly what I suspected was happening . When it cools off
and I get back to Memphis (I rode the Harley 350 miles round trip down
and back yesterday) to look into it more I think the first thing - after
testing current draw - after replacing the burned out transformer will
be to replace that condenser unit relay . And then feed the old one to
my son for not replacing the cover over the 'lectrics on the condenser
unit after he replaced the start cap .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell

On Sun, 14 Aug 2022 12:46:28 -0500, Snag <snag_one@msn.com> wrote:

>On 8/14/2022 1:18 PM, bud-- wrote:
>> On 8/12/2022 3:58 PM, Snag wrote:
>>> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
>>>> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>>>>> The furnace/AC system at our house in Memphis is going flaky . It has
>>>>> burned out the primary winding on 3 24V transformers ... The control
>>>>> board has been replaced , it doesn't blow the transformer when calling
>>>>> for heat , just AC .
>>>>> Our son that lives there has left the cover over the condenser
>>>>> relay/capacitor off after a start cap replacement , I'm wondering if
>>>>> rust in the relay might be the cause of the problem . What really
>>>>> puzzles me is why is it burning out the primary winding ? The fuse in
>>>>> the board is intact , the last transformer lasted about 20-30 seconds
>>>>> when we turned the AC on . Having already spent over a hundred bucks on
>>>>> parts already , I'm not inclined to spend more until I can figure this
>>>>> out .
>>
>> If the transformer connects to the thermostat it is "class 2", which is
>> limited power. I never tired shorting one, but I can't imagine only
>> lasting  30 seconds if shorted.
>>
>> I haven't seen an HVAC transformer larger than 40 VA. At 40VA that is
>> 1/3 A on the primary (plus something for inefficiency). Would be more
>> for a short, but it is limited power. Doesn't seem like much to burn out
>> the primary.
>>
>> 40VA  - at 24V that would supply 1.7A to equipment (more if shorted).
>> Measure current from transformer, particularly when AC is started
>> (temporarily)?  If you have a clamp-on ammeter, if you run the wire
>> around and through the window 10 times it will read 10 times the
>> current. You could get some similar ideas with low value fuses.
>>
>> Measure the transformer voltage when you start the AC?
>
> WE installed a window unit yesterday ... I'm not planning on going
>back up to that attic until the weather cools . And then probably only
>to insure the furnace is going to work .
>>
>>>>> --
>>>>> Snag
>>>>> “Free speech is my right to say what you don’t
>>>>> want to hear.” -George Orwell
>>>>
>>>>       Maybe you should unhook the wires right at the relay coil.
>>>> See what happens when you turn the
>>>> AC on.  Can you hook the relay to 24 volts from another source?  Fuse
>>>> it first.
>>>>
>>>
>>>    After repairing the bare wires and replacing the 2nd transformer I
>>> checked voltage at the AC condenser unit before hooking the wires to
>>> the relay . 24V on the nose . I don't really think I have another
>>> short in the control wire , I suspect excessive current draw on the
>>> relay because there is rust in there preventing it from completing the
>>> magnetic circuit . But I've still never seen the primary winding blow
>>> like these did . I'm riding down tomorrow (on my Harley) to help
>>> install a window unit I bought and had delivered . Gotta make sure
>>> it's done right ... he's too likely to take short cuts .
>>
>> If the compressor relay closes there is a continuous magnetic path and
>> the relay current is limited by the inductance. If the relay can't close
>> there is an air gap in the magnetic path and the inductance will be less
>> - higher coil current.  (AC relays close faster because of high coil
>> current until closed.)
>
> This is exactly what I suspected was happening . When it cools off
>and I get back to Memphis (I rode the Harley 350 miles round trip down
>and back yesterday) to look into it more I think the first thing - after
>testing current draw - after replacing the burned out transformer will
>be to replace that condenser unit relay . And then feed the old one to
>my son for not replacing the cover over the 'lectrics on the condenser
>unit after he replaced the start cap .
Sounds like Snag found something he can't fix or even jury-rig - It's
called "stupid". If it is fixable at all it needs to be started at
about age 4 - - - - -

On 8/15/2022 12:26 AM, Clare Snyder wrote:
> On Sun, 14 Aug 2022 12:46:28 -0500, Snag <snag_one@msn.com> wrote:
>
>> On 8/14/2022 1:18 PM, bud-- wrote:
>>> On 8/12/2022 3:58 PM, Snag wrote:
>>>> On 8/12/2022 1:51 PM, Dean Hoffman wrote:
>>>>> On Friday, August 12, 2022 at 12:06:05 PM UTC-5, Snag wrote:
>>>>>> The furnace/AC system at our house in Memphis is going flaky . It has
>>>>>> burned out the primary winding on 3 24V transformers ... The control
>>>>>> board has been replaced , it doesn't blow the transformer when calling
>>>>>> for heat , just AC .
>>>>>> Our son that lives there has left the cover over the condenser
>>>>>> relay/capacitor off after a start cap replacement , I'm wondering if
>>>>>> rust in the relay might be the cause of the problem . What really
>>>>>> puzzles me is why is it burning out the primary winding ? The fuse in
>>>>>> the board is intact , the last transformer lasted about 20-30 seconds
>>>>>> when we turned the AC on . Having already spent over a hundred bucks on
>>>>>> parts already , I'm not inclined to spend more until I can figure this
>>>>>> out .
>>>
>>> If the transformer connects to the thermostat it is "class 2", which is
>>> limited power. I never tired shorting one, but I can't imagine only
>>> lasting  30 seconds if shorted.
>>>
>>> I haven't seen an HVAC transformer larger than 40 VA. At 40VA that is
>>> 1/3 A on the primary (plus something for inefficiency). Would be more
>>> for a short, but it is limited power. Doesn't seem like much to burn out
>>> the primary.
>>>
>>> 40VA  - at 24V that would supply 1.7A to equipment (more if shorted).
>>> Measure current from transformer, particularly when AC is started
>>> (temporarily)?  If you have a clamp-on ammeter, if you run the wire
>>> around and through the window 10 times it will read 10 times the
>>> current. You could get some similar ideas with low value fuses.
>>>
>>> Measure the transformer voltage when you start the AC?
>>
>> WE installed a window unit yesterday ... I'm not planning on going
>> back up to that attic until the weather cools . And then probably only
>> to insure the furnace is going to work .
>>>
>>>>>> --
>>>>>> Snag
>>>>>> “Free speech is my right to say what you don’t
>>>>>> want to hear.” -George Orwell
>>>>>
>>>>>       Maybe you should unhook the wires right at the relay coil.
>>>>> See what happens when you turn the
>>>>> AC on.  Can you hook the relay to 24 volts from another source?  Fuse
>>>>> it first.
>>>>>
>>>>
>>>>    After repairing the bare wires and replacing the 2nd transformer I
>>>> checked voltage at the AC condenser unit before hooking the wires to
>>>> the relay . 24V on the nose . I don't really think I have another
>>>> short in the control wire , I suspect excessive current draw on the
>>>> relay because there is rust in there preventing it from completing the
>>>> magnetic circuit . But I've still never seen the primary winding blow
>>>> like these did . I'm riding down tomorrow (on my Harley) to help
>>>> install a window unit I bought and had delivered . Gotta make sure
>>>> it's done right ... he's too likely to take short cuts .
>>>
>>> If the compressor relay closes there is a continuous magnetic path and
>>> the relay current is limited by the inductance. If the relay can't close
>>> there is an air gap in the magnetic path and the inductance will be less
>>> - higher coil current.  (AC relays close faster because of high coil
>>> current until closed.)
>>
>> This is exactly what I suspected was happening . When it cools off
>> and I get back to Memphis (I rode the Harley 350 miles round trip down
>> and back yesterday) to look into it more I think the first thing - after
>> testing current draw - after replacing the burned out transformer will
>> be to replace that condenser unit relay . And then feed the old one to
>> my son for not replacing the cover over the 'lectrics on the condenser
>> unit after he replaced the start cap .
> Sounds like Snag found something he can't fix or even jury-rig - It's
> called "stupid". If it is fixable at all it needs to be started at
> about age 4 - - - - -
>

No Clare , it's called "lazy" . My wife says he's so much like her
father it's scary . Thing is he didn't learn that from us , we both
worked our asses off - often working 2 jobs to make ends meet - while
the kids were growing up .
--
Snag
“Free speech is my right to say what you don’t
want to hear.” -George Orwell